Why can't an airplane just fly into space? Why do we need rockets?

Some of our friends who are members of the Back Bay Astronomy Club in Virginia Beach, Virginia, often hear the questions "Why can't an airplane just fly into space? Why do we need rockets?"Well, most of the time air may not seem like a very real substance, like water or wood or metal. But when the wind blows so hard it nearly knocks you down, air seems very real indeed. If we think about it, we can begin to understand how essential it is to how airplanes work.

Airplanes are able to fly because air moving under their wings is strong enough to hold them up. If you could slice across an airplane wing, you would see it is curved over the top and flat on the bottom. As the plane's engines push the wing forward, air moves over and under the wing. Because the top of the wing is curved and the bottom is flat, the air going above has a little farther to travel than the air going below. The air molecules on top are thus a little farther apart, making the air there a little thinner, and the pressure on the top of the wing a little less than the pressure on the bottom. So what happens? The wing is pushed up by the air underneath it.

Large passenger planes can't fly much higher than about 12 kilometers (7.5 miles). The air is too thin above that altitude to hold the plane up. Some kinds of planes can fly much higher, and one special NASA plane, named Helios, flew to about 30 kilometers (19 miles), which is far higher than any other plane has traveled. At that altitude, the air is 100 times thinner than at sea level. The air gets thinner and thinner the higher you go, until there's hardly any air at all. In other words, it's nearly a vacuum up there.Even the lowest Earth-orbiting spacecraft orbit at around 200 kilometers (125 miles) above Earth's surface, far above the thick air we are accustomed to and much higher than any plane can reach. Most Earth orbiters, though, are placed at much higher altitudes to do their jobs. Some of these jobs include scientific observations of the universe that are better than scientists can make when looking through the air, as well as observing Earth's weather and long-term climate changes, taking pictures of Earth's changing surface, and studying pollution in the atmosphere.

To get a spacecraft to even the lowest Earth orbit requires a rocket. So how is a rocket different from an airplane? Rockets do not depend on air, even for burning their fuel. Rockets take advantage of some basic laws of nature that were discovered by the brilliant scientist Isaac Newton late in the 17th century. One of these, called Newton's third law, says that for every action there is an equal and opposite reaction. This explains what is going on when you blow up a balloon, then let it go without tying a knot. The air rushes out of the mouth of the balloon and that pushes the balloon in the opposite direction. This law also tells us that, to build a powerful rocket, we need to shoot out a lot of high-speed material opposite the direction we want the rocket to go. That is exactly what a rocket engine is designed to do. Most rockets use high-speed exhaust gases from burning rocket fuel to propel themselves up and away from Earth's surface to the vacuum of space. Unlike planes, they don't need air to lift them up.

Like everything else that burns, rocket fuel cannot burn without oxygen. Because it operates where the air is too thin to provides enough oxygen, a rocket carries its own oxygen in tanks and mixes it with the fuel just before it is burned.